The present invention relates to an equipment for continuous casting of strands of metals, preferably ingots of aluminium, comprising a flexible mould.
The casting of rectangular ingots commonly involves the use of moulds where the widest faces of the mould have a concave curvature. Such curvature is necessary to compensate for the shrinkage in the side surfaces under the casting operation. The amount of shrinkage will be proportional with the extension of the non-frozen metal in the strand after casting conditions are stabilized. During the casting of large ingots, the extension of melted metal in the lengthways direction of the ingot (marsh-depth) may be up to 0.8 meters.
It is primarily the casting speed that influences the extension of the marsh, because it is the thermal conductivity of the material that limits the cooling speed in the middle of the strand. The amount of water that is sprayed onto the surface of the ingot from the underside of the casting mould will represent a cooling capacity that goes beyond the amount of heat that is transported to the surface by heat conduction.
With respect to both metallurgy and productivity, it is desirable to apply the highest casting speed possible. The casting speed is normally limited by the tendency of heat cracks to form in the strand casted when the speed is too high.
In the initial stage of a casting operation the cooling will be slow and there will be a contraction in the strand casted caused by the difference in specific density between the melted and the frozen metal, together with the thermal coefficient of expansion. The metal that has frozen initially will be of a somewhat reduced shape with respect to the geometry of the casting mould. Because of the above mentioned curvature of the widest faces of the casting mould, the strand casted will have a convex shape in the initial stage of the casting operation. The convexity will gradually reduce until stable conditions with respect to the marsh-depth in the strand casted are established. The rolling mills specify that the rolling surfaces should be straight and planar (i.e. without any concavity/convexity in the rolling surfaces). To meet this requirement, the casting moulds have to be designed with an amount of flexing (curvature of the widest faces) that is related to the expected shrinkage/contraction.
The lowest part of the casting strand has a defined convex cross-cut that is commonly recognized as the butt end. The extension of the butt end is mainly determined by the amount of flexing in the respective casting mould. Typically, the extension may vary from 20 centimeters to 80 centimeters depending on the dimensions of the strand casted and the amount of flexing. The part of the butt end that will not satisfy the specifications of the customer has to be cut off by the ingot producer and represents a substantial part of the scrap produced in the casting process.
As mentioned above, it is mainly the casting speed that determines the contraction, and a casting mould will therefore render an optimal ingot geometry for a certain speed. In other words, a casting mould designed for a high casting speed will produce a convex ingot when casting at a lower speed than the design speed. On the other hand, casting speed that is too high with respect to the designed speed will give concave rolling surfaces.
To optimize the return from the casting process and to reduce the geometrical deviations of the strands casted, there have been developed casting moulds with flexible wide faces.
U.S. Pat. No. 4,030,536 discloses a casting mould for continuous casting of ingots of rectangular cross-section. The narrow faces of the ingot are arranged in such a manner that their mutual distance is kept as constant as possible, while the wide faces are flexible. As the casting speed increases, the distance between the middle parts of the wide faces is gradually increased. According to the example disclosed, the distance between the wide faces of the mould is adjusted by means of a flexing mechanism comprising a manually-actuating screw jack device 16 arranged at the outside of each wide face. Each screw jack device is connected at one end with a rigid frame section at the outside of the mould, and connected at the other end by means of a yoke and two hinged connections with the wide face of the mould. This attachment of the yoke will cause the inner surface of the mould to have an even, concave shape as the jack is tensioned. Thus, the maximum value of the distance between the wide surfaces of the mould will be apparent as the distance between the hinged connections at each side. The presented solution further comprises a cooling system that chills the strands as they are casted. The cooling system comprises an upper and a lower channel for coolant water surrounding the mould, at a short distance from the mould, where the channels have orifices that sprays coolant water respectively towards the walls of the mould and the strand casted.
One disadvantage of this embodiment is that it requires an active follow-up by the operators for the control of the mould flexure versus the changes of casting speed, as long as the part rejected should not become too comprehensive. One another disadvantage with this solution is that the even, convex shape of the wide faces contributes to the rejection of at least one first part of the ingot casted because it does not satisfy the required tolerances set by the customer.